The present invention relates to construction and method of manufacturing of electronic components such as infrared sensors for use, for example, in thermometers and human body detecting sensors, and piezoelectric devices for use, for example, in acceleration sensors and the like. In particular, it relates to a method of manufacturing sensors which are useful for electronic components in general in which circuit structures required for impedance conversion by using field effect transistor elements and the like have been mounted, and resistive elements for use in these sensors
There exist many electronic components such as various types of sensors which require impedance conversion by means of field effect transistor elements and the like when outputting signal after amplifying a minute signal generated by the sensor. FIG. 5 shows an impedance conversion circuit employing a sensing element 51 and a field effect transistor element 53.
In the impedance conversion circuit of FIG. 5, sensing element 51 is connected in series with a resistor 52, and the output of sensing element 51 is connected to the gate of field effect transistor element 53. In a sensor generating a very small electrical signal, (e.g.,resistor 52) with a rather high resistance value ranging from several tens of Mohms to several Tohms is used in many cases depending on the type of sensor.
FIG. 6 shows a view illustrating the mounting on a stem of the circuit of FIG. 5. Four components are to be mounted, namely, a sensing element 61, a resistive element 62, a field effect transistor element 63, and a mounting board 64. In this configuration, the surface of a stem 65, which is the substrate for mounting, is covered with a metal, which operates as a grounding (GND) electrode 66. This is for the purpose of providing protection from external noises. Field effect transistor element 63 is formed on a silicon substrate, and an electrode operating as a gate terminal (G) is formed on the rear surface of the chip whereas a source electrode (S) and a drain electrode (D) are formed on the top surface of the chip.
From the standpoint of circuit configuration, it is impossible to dispose field effect transistor element 63 directly on stem 65. Consequently, it is first connected via a conductive resin 67 on mounting board 64 which is provided with a connecting electrode on the top surface. While bonding of components other than field effect transistor element 63 does not require a conductive resin, the same conductive resin 67 is generally used for simplifying the mounting process. The connection between electrodes of each elements and substrate electrodes is performed by wire bonding using thin metal wires 68 of Al or Au, and then to an external terminal 69.
As has been described above, the conventional canned package has as many as four components that need to be mounted, calling for six wires to be wire bonded. Accordingly, the use of many components means not only a high materials cost but also an increase in man-hours for the mounting process resulting in a high cost. Also, as a space is required on the stem, a larger stem is required, thereby making the sensor size larger.
The present invention addresses these issues and aims at realizing a lower sensor cost as well as providing a smaller package.
In addressing the above issues, the present invention provides a method of manufacturing a sensor comprising a resistive element having a top surface electrode and a bottom surface electrode, a sensing element generating an electrical signal by sensing energy from the outside, a field effect transistor element on the rear side of the chip of which a gate electrode is formed, and a substrate having a first electrode, a second electrode, and a third electrode on its top surface, the method comprising the steps of electrically connecting the bottom surface electrode of the resistive element and first electrode of the substrate, electrically connecting the field effect transistor element onto the resistive element so that the gate electrode and a portion of the top surface electrode of the resistive element meet, electrically connecting one of the electrodes of the sensing element and a portion of the top surface electrode of the resistive element, electrically connecting the source and drain electrodes of the field effect transistor element respectively to the second electrode and the third electrode on the substrate, and electrically connecting the other electrode of the sensing element to the first electrode on the substrate. With this invention, the mounting board of the prior art example becomes unnecessary and the number of wires is reduced from 6 to 4 thereby reducing the cost. Also, as a resistive element is disposed at a position where a mounting board is disposed in the prior art, the space occupied by the resistive element becomes unnecessary thus providing a smaller package.
The present invention also provides a sensor in which the resistive element of the above described sensor is formed with a ceramic material, glass material, or the like, or ferrite material, and in which a resistor body having a relatively high resistance value in the range from several tens of Mohms to several Tohms can be formed with ease.
The present invention also provides a method of manufacturing a resistive element having a predetermined resistance value by forming in advance an electrode over the entire top and bottom surfaces of a flat resistor body with a large area and cutting to arbitrary dimensions after measuring the resistance value. According to this invention, as the resistance value is inversely proportional to the area of the electrode, it is possible to change the resistance value by the size to be cut based on the resistance value measured in advance when the area is large, thereby allowing formation of a resistive element having a precise resistance value which is intended to be obtained after cutting. Also, by changing the area to be cut, it is possible to obtain resistive elements having many types of resistance values from the same resistor body. Furthermore, it is a method of manufacturing with superiority in mass producibility as it is possible to configure a continuous mounting process from cutting to mounting by employing a cutting method such as dicing or the like. The present invention also provides a method of manufacturing resistive elements in which the resistor body to be used for the resistive elements is formed at a sintering temperature such that the rate of water absorption becomes 1% or less. According to this invention, even when a process in which cutting of the resistor body to be used for resistive elements is diced while water is being sprayed, no change in the resistance value due to water absorption or moisture absorption by the resistor body occurs. It is also possible to realize a high reliability resistive elements as no change in the resistance value is caused even under a high-temperature, high-humidity environment.
Furthermore, in the present invention, the method of manufacturing the above described sensor is one in which the resistive element is simply a resistive element of which the electrodes are formed on the top and bottom surfaces of the resistor body, a first electrode on a substrate is electrically connected with the bottom surface electrode of the resistive element via an conductive material, and a predetermined resistance value is obtained by controlling the amount of the conductive material so as to control the amount of resin which rises on the sides of the resistor body. With this invention, it is possible to correct the resistance value by controlling only the amount of resin to be coated even when there is some variation in the obtained resistance values among production lots.
The present invention also provides a method of manufacturing a resistive element having a predetermined resistance value in which the resistance value is controlled by heat treating a resistive element of the above described sensor in a vacuum, in a reducing gas atmosphere, or in an inactive gas atmosphere after forming the top surface and bottom surface electrodes. According to the present invention, as it is possible to change the resistance value of the resistive elements fabricated by the same method of manufacturing over a wide range, it makes it possible to configure resistor bodies having many types of resistance values from a resistor element fabricated by the same method of manufacturing. It is also possible to change the resistance value of a resistor body after mounting.
In the method of manufacturing the above resistive element, the present invention provides a method of manufacturing in which resistive elements are fabricated by heat treatment in the atmosphere or in an oxygen atmosphere after heat treatment in a vacuum, reducing gas atmosphere, or inactive gas atmosphere. According to this invention, it is possible to fabricate stable and high-reliability resistive elements of which the resistance value does not change during mounting of the above-configured resistive elements or during heat treatment after mounting.
Furthermore, in the present invention, the electrodes to be formed on the top and bottom surfaces of a resistive element are made of a metal containing either of chromium, tin, or indium. According to this invention, even when a resistor body formed with a ceramic material, glass material, or ferrite material has a resistance value close to that of an insulator, it is possible to further widen the variable range of the resistance value in the above configured resistive elements by employing a metal containing either of chromium, tin, or indium as the electrode of the resistor body.